Cascaded converter cells, often denoted multilevel converter cells are of interest in many power transmission applications, for instance in High Voltage Direct Current (HVDC) Transmission.
These cells provide discrete voltage levels that can be combined for conversion between AC and DC. Each cell is here made up of an energy storage element, typically a capacitor, being connected in parallel with one or two switching element branches for forming a half or full bridge converter cell. These cells typically have two connection terminals, where a first is provided in the junction between the two switching elements of a first branch and a second, which in the case of a half bridge cell, is placed in the junction between one of the switching elements and the energy storage element and in case of a full bridge cell between the two switching elements of a second branch. The placing of the second terminal in the half-bridge cell defines the cell type, where it may be placed at the junction between one of the switching elements and the energy storage element. The placing of the second terminal at a first such junction therefore defines a first type of half-bridge cell, while the placing of the second connection terminal at a second junction defines a second type of cell.
Half-bridge cell based voltage source converters are generally described in DE 10103031.
In the case of disconnecting a converter for repair and/or maintenance or for protective purposes, the cell capacitors have to be discharged. This discharge should be fast and also safe, in order to guarantee the safety of personnel performing the repair/maintenance.
Such cell capacitor discharge is for instance described in JP 2009-247186, which document shows an inverter including cells with capacitors as well as discharge resistors. This document thus describes that each cell has its own discharge resistor, which is placed in parallel with the capacitor. The problem with this is type of discharging is that there will be some losses also in normal operation, which is disadvantageous, especially if power is to be transmitted over long distances, where a high efficiency is needed. The discharging may also be slow.
JP 2007-312456 discloses a discharge arrangement for another type of voltage source converter. However, the converter is not cell based. JP 2007-312456 discloses a power converter where there are a number semiconductor devices and one intermediate capacitor in parallel with all semiconductor devices. There is also one discharge resistor in parallel with each semiconductor device. Since also this document describes the use of resistors in parallel with the switching elements of the converter, there are also in this case unnecessary losses.
In view of what has been described above there is therefore a need of enabling discharge of the cell capacitors in a cell based voltage source converter that minimizes the losses in normal operation.